“[...] based on the fact that these mutants have a super high concentration of the virus, it stands to reason that there could be some place which is creating them. As to where they could be, I cannot hazard a guess.”— Vree's autopsy report

“My job was still not finished, for I still had one task remaining. The Master had literally built his army one mutant at a time. Humans, preferably with little radiation damage, were to be captured and sent to the Vats.”— Vault Dweller's memoirs

The construction of Mariposa started in October 2076 when significant progress was made in FEV research, and the government preferred to move the project to a location under military supervision

In 2077, Major Barnett ordered transfer of all FEV research to the newly-constructed Mariposa Base, despite objections from the research team. Along with the scientists, a military team commanded by Colonel Robert Spindel, which previously monitored the experiments at West Tek, was sent to Mariposa. Spindel's second-in-command was Captain Roger Maxson. The FEV project was moved to Mariposa, where tests on human subjects were performed. The vats were giant tanks filled with liquid FEV and as part of the research project, human subjects were dipped inside them, then examined to learn how to control and harness the process. The experiments on humans were kept secret even from Spindel and his soldiers.

On October 10, Maxson and his men were horrified to discover that the scientists at Mariposa were using military prisoners as test subjects in their experiments with the Forced Evolutionary Virus. Morale in the base broke down and Spindel suffered a mental breakdown, eventually committing suicide 5 days later. The men turned to Maxson's for leadership.

After interrogating Robert Anderson, the chief scientist, and learning about the extent of their actions, Maxson executed him and the other scientists soon followed. Maxson, now in control of the entire base, declared himself to be in full desertion from the army via radio on October 20. However, he received no response, as the rest of the army was busy fighting the Chinese threat. Three days later, the bombs were launched, and the Great War ended two hours after it started.

The Mariposa Military Base survived, the soldiers within protected from the radiation and FEV flooding the wasteland. Two days later at Mariposa, a scout in power armor named Platner was sent out to get specific readings on the atmosphere. He reported no significant radiation in the area surrounding the facility. After burying the scientists in the wastes outside of Mariposa, the soldiers sealed the military base, then headed out into the desert, taking supplies and weapon schematics with them. Captain Maxson led his men and the squads families to the government bunker at Lost Hills, eventually forming the Brotherhood of Steel.

The base was left sealed, however, over the years it slowly fell into disrepair. Eventually various creatures made their way inside, mutating due to FEV exposure, then proceeded to infest the surrounding area, attacking fledgling human settlements and caravans. The problem was becoming increasingly severe, to the point when Harold, a caravan master from the Hub, mounted an expedition in 2102 to locate and destroy the source of the mutants.

Dr. Richard Grey was one of the explorers who was part of the expedition. Harold led the men into Mariposa, but failed to account for the automated defenses, which were still online, which promptly attacked the explorers.

Taking heavy casualties, they made their way to the deepest, fourth level, the FEV vats, where a crane smashed into the two surviving explorers, Harold and Richard Grey. The former was knocked out, the latter landed in one of the virus vats, succumbing to a severe overdose.

In pain and horribly mutated, Richard crawled into the control room, as his intellect grew. In a few days, he was neurolinked to the base computer, drawing upon its sizable data banks. As he created the idea of the Unity, wanderers began to enter the facility, whom Grey patiently captured and dipped, in order to create beings as perfect as him.

When the first true super mutants were created, he set out to create a true mutant army, at the same time deciding to look for a permanent base of operations. In 2155, one of his patrols located a group of vault dwellers from the LA Vault, and he made the decision to transfer himself there, occupying it.

The base was entrusted to the Lieutenant, one of the greatest super mutant minds to exist. Mariposa was rebuilt and cleaned up, once again becoming the magnificent military base it was before the war. Super mutants roamed the halls, restocking the armories and medical base and soon it became the staging area for their assault on humanity, as well as the only super mutant production center.

Then came the Vault Dweller in 2162. It is unknown whether or not he infiltrated the base or blasted his way in - suffice to say, once he was through, Mariposa was little more than a caved-in, smoldering ruin.

In 2236, the Enclave discovered the remnants of the Mariposa Base. Soon, assault squads combed the desert for slaves they could use to mine the military base and get to the vats. One of the squads included soldier Frank Horrigan.

Enclave construction crews, as well as human slaves began excavations. In September, they uncovered the FEV and mutations began to occur in the human workers. Frank Horrigan came into contact with the virus and was sent to the Enclave labs for study.

In 2237, having obtained the FEV data, the Enclave abandoned the site after more mutations occurred, causing 2nd Generation super mutants to arise. The Enclave left a single squad behind to wipe out the super mutants, but the mutants, using armaments they had cached in the base during excavation, reduced the squad to ashes after suffering heavy casualties.

By 2241, the remains of the ruined base are inhabited by those trapped human miners who have been hideously altered by the remnants of the FEV that still contaminates the wreckage. That group formed a new community under the leadership of Melchior, whose magical talents had allowed them to hide the weapons.

Mariposa was a reinforced military base, constructed in the middle of nowhere, in the side of a mountain. The exterior had a characteristic white lining and a massive air conditioning vent, which provided fresh air for the complex. Inside, it consisted of four levels, stretching across a mile of depth.

The first level of Mariposa housed the gear needed for any excursions, as well as a staging area for patrols. The main control computers for the base's force fields and robotic guardians were located here, allowing any infiltrator to considerably sabotage the base, just by altering the pest control setting for the robobrains, in addition to altering the force field status remotely. Apart from that, guard barracks were located on this level, for off-duty surface security teams.

The primary soldier living quarters, medbay and armory were located on this level. The quarters were spacious, although spartan in design, with a large stockpile of weapons just around the corner and a well equipped sickbay, stocked chock full of medical chems, surgical equipment and medical beds with integrated life support systems.

The third level was divided in two, one was the general access area, where the assembly room and elevator to the commander's office were located, the second a military prison with an elevator going directly to the vats. When Mariposa was operating under government jurisdiction, experimenting with FEV, only scientists and their security staff were allowed into the prison area and the vats - soldiers under Colonel Spindel and Roger Maxson were barred from entry.

Divided in two as well, the deepest level housed the massive FEV vats, together with their control room, scientist living quarters as well as the base commander's office. As with the third level, access to the FEV research area was strictly limited, with only scientists being allowed to enter. A special containment squad, the super mutant Vat Team 9, was stationed in the room near the cell block access elevator, to deal with any potential disturbances. It was composed of three soldiers.

The vats control room was where the base's mainframe was also located, holding personnel logs (Boyarsky, Roger Maxson, Richard Grey etc.) as well as the self-destruction initiation subsystem. Encrypted behind a firewall, these were nonetheless accessible for a skilled hacker. Alternatively, just blowing the console up would trigger the self-destruct sequence, but the base would be set to alert status, powering up the force fields.

The layout of the base is pretty much similar to that in Fallout, although there are some tents (in one of them is the Military base holodisk 5) outside the base, wolf packs and the entrance to the base is blocked by rubble. To get in the base the player character will need to find the metal pole and put it on the rail cart ("Use on"). Next, they need to put some dynamite (can be found in the little shack right from the entrance to the base) on the rail cart (one need to right click and hold on the cart, and then choose the "bag" option; after that, simply click on the dynamite from the menu) and push it down. Doing this will net the player character 5,000 XP.

When the player character arrives to the first level of the base, first thing they would notice are the Enclave soldiers and mutant bodies lying around. The player character can loot these, for holodisks and a plasma pistol.

The level is mostly filled with mutated rats (which can poison the player character if they bite), which can be easily dispatched. However, in one of the halls, there is a super mutant, Grundel, which will attack the player character, if they try to talk to him. He is wearing spiked knuckles.

In the southwest corner (in the hall leading to the elevator) there is a broken power generator, which can be repaired for 1,500 XP (this can be easily done if one has Vic in your party). Doing this will also turn on the lights in the base.

Like on the second level of the base, the third level is also inhabited by super mutants which use laser rifles, plasma rifles and flamers. Also, there are some mutated beasts (pig rats) which are weak, but the player character can get poisoned if they bite them. There are super duper mutants on this level, which have greater HP than "normal" mutants. As comes to the loot, the player character should pay attention to the first room after exiting the elevator. In the room is the locker, in which the player character can find a green memory module, which can be used to expand ACE's medical library, thus allowing the player character to increase their Perception by 1. Finally, in the southeastern corner of the map, there is an elevator which takes the player character to the final level of the base, the vats.

The deepest level of the base houses one of the tougher enemies in Fallout 2, Melchior. He is standing in the northwestern corner of the map, surrounded by goo which has remained there since the Vault Dweller destroyed the FEV vats.

Melchior himself is not that tough, but his abilities are unique. From the vats (or more precisely, the goo), he can and will summon deathclaws, fire geckos, floaters and mutated mole rats. They are summoned in that descending order, and if the player character is strong enough to survive several waves of them, they are rewarded with some funny lines. He can have 4 summoned allies at a time, which can be bothersome, so it is best to dispatch him first, and then his "pets." This is quite challenging, even for experienced player character's with a good party, so they should be very cautious, and prepared to leave the scene of battle with 1-2 less companions. Melchior also guards a locker containing a PPK12 Gauss pistol.

In 2241, there are some super mutants named as super duper mutants. One of them can talk about their recipe: rat surprise. The recipe is told as when the rat is caught, its tail is needed to be left on a wall until it falls down. Once done, it is needed to be eaten quickly, else "the surprise would crawl out and fly away."[1]

According to Chris Taylor, Mariposa Military Base is based on Fort Ord, an old military base that has been shut down near Monterey Bay. It was called Base Omega in early design documents.

The base is rather far away from the real-life Mariposa County, which is on the other side of the California Valley, close to the in-game location of Vault 13. It's also much further north than Fort Ord. The closest real-life locations are the San Luis Reservoir and the city of Los Banos, CA.

The name of the base, Mariposa, in Spanish means butterfly. This connects to the metamorphosis that occurs in a caterpillar turning it into a "better" creature, in this case a butterfly. When pure humans are dipped into the FEV vats they transform into a "better" creature, in this case, a smarter/faster/stronger super mutant.

In a few cases the yellow force fields' device cannot be deactivated by the use of a tool. If that happens it's only necessary to simply use the Repair skill directly into the force field device. [verified]

In Fallout, in Mariposa level 2, there are some super mutants locked in a room. Loading a previous save will make them all dead, or make them disappear entirely. [verified]